Pulse analyzer



M. J. POOLE PULSE ANALYZER pril 28, 1 953 zsHBErs-SHEETl .EQAIZR /EIZLSEEEN Filed May 20, 1948 MFA/661350625 F/GZ.

/ (OUA/ERS nscR/M/f/Am's Inventor MMM/? Alterna/yl:

M. J. POOLE PULSE ANALYZER April 28, 1953 42 sms-SHRET 2 Filed May 20; 1948 FIGS.

fg R21 l l l I l I -J Inventor JYLJPO 0 Le A ttomey Patented Apr. 28, 1953 PULSE ANALYZER Michael J. Poole, London, England, assignor, by

mesne assignments, to National Research Development Corporation, London, England, a corporation of Great Britain Application May 20, 1948, Serial No. 28,122 In Great Britain June 14, 1947 (Cl. Z50-27) 8 Claims. 1

This invention relates to pulse analysers for classifying the pulses of a train into groups of different amplitude ranges.

One known method of analysing a pulse train, such as the amplied pulses from an ionisation chamber subject to particle bombardment, is to photograph the pulse train in a recording oscillograph and subsequently to measure the individual pulses. This method suiers from the disadvantage that the analysing process is a long and tedious one, particularly if a high statistical accuracy is required and in the case of a mechanical oscillograph it is limited to counting speeds of a few hundred per minute.

The invention will be described with reference to the accompanying drawings wherein- Fig. l is a block diagram of a known form of pulse analyser;

Fig. 2 is a block diagram of a pulse analyser embodying the invention;

Figs. 3 and 4 are wave form diagrams; and

Fig. 5 is a circuit diagram of my embodiment of the invention.

Another method is to employ electronic means to distribute the pulses according to their arnplitude. They may then be counted automatically of otherwise utilised. A known pulse analyser based on this method is shown diagrammatically in 1 of the accompanying drawings. In this ligure D l D2, D3, D4 represent discriminators or relays of graded back bias to all of which the pulse series to be analysed is applied through an amplifier A providing a suitable initial amplitude level. Counters C are associated with successive pairs of discriminators to count only if the rst discriminator of the pair has responded to a pulse while the second of the pair has not. If for example, discriminators Di, D2, D3, D4 have bias voltages of 0, 10, 20, 30 respectively and operate to apply a negative pulse to the preceding counter and a positive pulse to the succeeding counter, all output pulses being equal, then on the arrival of a 23 volt pulse from amplier A discriminators DI, D2, D3 operate to apply to their counters pulses of sign as indicated in the figure but discriminator D does not operate; the pulses on Ci and C2 neutralise each other so that C3 alone is excited to count that particular pulse from ampliiier A. It will thus be seen that, in the example referred to, counter Cl registers pulses in the voltage range 0 to 10, C2 registers pulses in the voltage range 10 to 20 and so on.

Arrangements of the above-mentioned kind give rise to difficulties because the pulses from 2 the discriminators are not coincident, this tendency being due to the iinite rising time of the input pulse.

In accordance with the present invention there is derived from the initial pulse series a train of shorter pulses of greater uniformity in amplitude and waveform and thesederived pulses are distributed to counters or other output channels according to the amplitudesl of the corresponding initial pulses.

The distribution may be effected by means of discriminating electronic switches or gate circuits controlled by the initial pulse series.

Referring to Fig. 2 derived short pulses are applied to electronic switches or gating circuits GI, G2, G3, G4 the opening and closing of which are controlled by discriminators Dl, D2, D3, D4. These discriminators of graded back-bias have the initial input pulses applied to them and when one of them is excited by a pulse of suiicient amplitude it tends to make the preceding gate circuit inoperative and the succeeding gate circuit operativeto pass the short pulse to the corresponding counter. v

The discriminators and gates circuits thus function to make a path'for each derived short pulse to reach its appropriate counter and to block the paths to other counters.

One convenient form of gate circuit is comprised by a thermionic valve having three grids, to the second of which the preceding discriminator applies a positive pulse tending to accelerate electrons to the anode and to the third of which the succeeding discriminator applies a negative blocking pulse. Current through the valve is normally blocked by a negative bias on the iirst grid so that the discriminator pulses are not passed 4to the counter.

Thus if D l, D2 and D3 only are excited, the gate valves in GI and G2 are cut o by pulses on their third grids, the gate valve in G-i has an inelective second grid voltage since no pulse arrives from D4 while the gate valve in G3 is given second and third grid voltages suitable for current flow through it. No current does flow however, until the short derived pulse arrives to overcome the blocking bias on the iirst grid. Thus it will be Seen that the discriminators have eiect to prepare the path to the counters but not themselves to excite the counters.

Inlig` 3 videallised pulse forms are shown of an initial input pulse series, and of the consequent pulses at the electrodes of a gate valve such as that described above. A iirst discriminator DI is assumed to have a back-bias of 10 volts and to be excited by the second and third pulses; a second discriminator D2 is assumed to have a back bias of 15 volts and to be excited by the third pulse. Short derived pulses arriving at the rst grid are shown at Vgl. D-iscriminator D1 applies positivefpulsesi Vee to the secondi or screen grid; D2 applied ainegati've puise Vex. to thethrd' grid?, Under these conditions the valve is operative only during the second short pulse and the reg sulting anode voltage is as shown at VA.

Fig. 5 is a circuit diagram of a pulse analyserof the general form of Fig. aand inclndesapulse generator T for deriving shortzpulsea. discrimina... tors DI and D2 including phase. reversingF valves VHS and V25 for providing positive pulse outputs.. and gate circuits GI and G2.. Succeeding dis.- criminators and gate circuits are not shownbutare similar to DI, D2 and Gl G2 respectively.

There is also preferably included a grouping discriminator GD to the. input of which the initial; pulse train to; be.;A analysedis. applied and from; which any-'desired part. of the amplitude range may oe abstracted and applied as input to-the-:discriminators Di.,.D2 etc. In.. the absence of. such agrouping discriminato-r, the initial pulse train. is; applied directly appropriate sense to the discriminators DI., D2 etc.

Before it ispossi-bleito. explain thecounting circuits used in` this particular pulse analyser it is necessary to understand the Working. of the discriminators employed. and these; are therefore described indetail iirst. They are amodiication of the Schmittthermionic; trigger mourn. of Scientic. Instruments,` 15. 24, 1938i which basically comprises two` valves like-.VI.4,. VIVE in. Fig. 5. with acorn-men: cathode' resistance Rill, an output resistance network .RI-.5, RIVE. RIf'I. for valve VIA with a.. direct coupling to valve V153:-yv and an anode: load` resistance RIS for valve VI 5..

Normally ther grid of VI4- is maintained aty a positive potential sutficient: to cause V15. to be cut-v off. Now suppose. the.- grid. potential of VIA falls until VIe5 starts to, take, current. Then the system will become unstable 'when thetotal gain round. the; loop.: formedby the. grid.; andf cathode couplings oi the valve exceedsunityf. The current through VI|5T then suddenly increases to a value determined enti-relybyits cathode resistor Rift and.` grid potential, While V-I-4 cuts ofi. Thisstate stableas long as the grid of VN does4 not rise above anotherl critical. potential' atv which.- VM again conducts.` The two. critical potentials (on and ofi); are.l sepa-rated' by an. overlap orY back-- lash voltage of anything from a fraction of. a volt to a few volts; dependingr on circuit conditions. As will be seen later this eitect is very valuable in the working of. the discriminator.

The adopted circuit of thevdiscriminator di'ers from the basic. Schmitt trigger circuit. by the addition of a grid condenser CIB in shunt to the coupling, resistance R16- and a condenser CIA across the common cathode resistor Rlll.` The former serves to improve the gain at, high frequencies and so speed up the trigger action, while the latter has been found experimentallyI to. increase the speed of the initial fall of output potential (though slowing the. rise later) and as will be seen later the successful operation of the counting circuits depends on. this fall being fast` An important point in the design of a. practical pulse. analyser is the stability oi the voltage at which. each discriminator res.. This depends largely on the stability of the Values of resistances RIS, RIE and. RII (which are. preferably on this account wire-wound), and. on the stability of the supply voltages. It is noteworthy that if the positive bias and the H. T. voltages are derived from the same source partial compensation of voltages variations takes place and eiectively onlyY variations in the cut-off bias are observed.

These.. discriminators. have a measuredresolvirlgjme of item5 to l0r secg..and.in virtue of the overlap in the two critical voltages are insensitive to noise superimposed on the pulse as long as the peak-to-peak noise voltage does not eX- ceed the overlap Voltage. This overlap voltage servesthe additional purpose of ensuring a finite ontputpulse even. when the discriminator is only just. triggered; by` the peak of the impulse. The

' output. voltage; amplitude of the discriminatcr whenv connected to the following circuits is about 2 ,volts. The discrilninators DI, D2, etc. are coupledv to the gating valves VI'I, V2I etc. of gate clrcuitsGll, G2 etc., so that when a discriminator (say D2) res it passes a negative pulse to the suppressor of the gating valve Vil immediately before itin the sequence, and a positivey pulse to the screen of the gating valve V2.I immediately following it.. These discriminator pulses alone will not cause any oi the'valves Vil,A V21. etc.. to give an output pulse as they each have a large negative bias on the control' grid. Accordingly av sharp positive pulse from trigger unit T isapplied to. the. control grids of. all the gating valves at. a moment. when the pulsesv irom all the discriminators overlap.. This will: cause a pulse of c urrent to flow through that. gating valve whose screen voltage has risen, but which has not. had a negative pulse on its suppressor grid, that is the one .correspoi.idlrlliI to, the last. discriminator in the sequence. than has fired., This is the pulse that is passed on to the counting circuits. The only time when the pulses from all the discriminators.. overlap is. a. period. of about 1 u S60. from the peak of the input pulse, and the trigger pulse is. made to coincidewith the peak of the input pulse... As in. practice there. S always o tendency for it to come a little late. rather than early, this gives a pulse at the right moment. It will be seen now Why it is that the fact of the overlap voltage making a finite length pulse, from4 an input pulse that just triggers the discriminator, is so important, as this is the permissible interval within which the trigger pulse may lie.

Each gate circuit GIi, G2 comprising gatingvalve VJIT, V21 has its screen grid voltage determined by a. phase-inverter valve VIE, V20. Referring specifically to gate circuit GI, valve V'I 'l is normally cut off by a large positive cathode bias developed across a resistance RIS and in addition its screen grid is maintained at a low voltage by the anode current taken by VIE. The discriminator DI immediately preceding the gating circuit is connected to the grid of VIE and when it fires the negative square-wave produced cuts oil? VIE and so allows the screen of VI'l to rise to a normal Working voltage. It is desirable to apply to the grid of VIS only a part of the full output from the discriminator .to prevent spurious pulses from the suppressor grid of the preceding gating unit being fed to this grid at the moment of the trigger pulse. The potentiometer chain. RZI, R22 shown to the grid of VIS is arranged to apply the necessaryA cut-oil voltage (4 or 5 volts) from the discriminator and this re duction of the spurious pulses eliminates the trouble. The discriminator D2 immediately following the gating valve Vil is connected directly to its suppressor grid and s0,.if this discriminator fires, it once againrenders the gating valve insensitive.

The whole circuit arrangement of Fig. 5 is designed to use a common type of valve throughout, namely a high-slope high-frequency pentode such as type EF50. Specically this type is used as gating valve. About l() to volts output pulse is enough to trigger the following counting circuit and the valve characteristics are such that if the gating valve normally has zero suppressor grid bias the volts output from the discriminators will not be enough to render the valve insensitive unless the trigger pulse amplitude is very carefully adjusted. To get over this diniculty the suppressor grid is arranged to have a standing bias of -20 volts (and so swings from -20 to -40 volts). Under such conditions, the operation of the circuit is nearly independent oi the trigger pulse amplitude provided that this is not large enough to cause grid current to flow. Rather than arrange that the control grid has a different bias from the suppressor grid (which would involve complicating a large number of units) the control grid is also given this bias and the trigger pulse is made large enough to overcome it. The bias may therefore be applied to the cathode with respect to ground and this is done by connection of the cathode to the potentiometer chain RIS, R20. Y

As was previously mentioned the successful operation of this circuit depends on the steep initial fall of potential from the discriminators. This is to ensure that all the gating circuits have taken up their nnal sensitive or insensitive conditions before the arrival of the trigger pulse. It will be seen that granted this the operation of the circuit is independent of the shape of the pulses being analysed and depends on their amplitude only.

The output pulses from each of the gating valves Vl, V21 etc., is fed into a scale-of-4 or scaleof-ten counting circuit operating an output valve and counting meter.

The trigger pulse applied to the control grids of the gating valves from trigger pulse generator T is a positive pulse of about 2 ,u sec. duration and volts amplitude, which is arranged to be initiated exactly at the moment of the peak of the input pulse. This pulse is generated by difierentiating the input pulse with respect to time and making use of the fact that at the peak of the pulse the diiferential coenicient passes through zero. To do this the differentiated pulse is applied, after passing through suitable amplier and limited circuits, to a Schmitt trigger identical with these in the discriminators biassed so that it falls back at zero input volts.

efore examining the circuit in detail it is necessary to consider the input pulses that the unit has to handle. In the present example, these will range from 3 to 100 volts in amplitude and will be, if a certain simple condition in the input amplifier is satisfied, similar in shape. This condition is that the collection time of the ions in the ionisation chamber or counter connected to the amplier shall be much shorter than the response time of the amplifier to an instantaneous rise. The rising time of the pulse will then be controlled entirely by the anfplier, and the falling time is in any case only dependent on the coupling time constants in the amplier. Thus the amplitude range of the differential coeicients is the same as that of the pulses.

The detailed circuit of the trigger pulse unit T, Fig. 5, will now be'considered with'reference to the pulse forms of Fig. 4. The input pulse form is as shown at a Fig; 4. Valve V8 serves to cut off any negative tail of the pulse and so obviate the possibility of a second zero in the differential coefficient; the output pulse from this stage appears at the cathode of V6 as shown at b, Fig. 4.

A small adjustable negative bias is provided to allow suppression of amplifier noise if required. The pulse is then differentiated by the combination RSCG to give at the grid of valve Vi the waveform c, and it is found satisfactory in prac'-` tice to arrange the values of R6 and CS so that the amplitude of the differentiated pulse is 1,50 to 1/40 of the input pulse. As diiferent shapes of pulses may have to be handled C6 is made variable by means of a switch. It is necessary to adjust CS (to Within a factor 2) to suit the ampliiier in use. Valves Vl and V8 now serve to amplify and limit the differentiated pulse, the pulse form at the grid of valve V8 being shown at d and that at the grid of valve V9 being shown at e in Fig. 4. It is obviously only necessary that the amplication should be distortionless in the neighbourhood of zero volts, but it is quite essential that at no time` should grid current flow as this will displace the moment of zero volts. The amplier has to amplify the smallest diiferentiated pulse (about fg volt in the positive direction) up to the 5 volts required by the Schmitt trigger, without being so overloaded by the largest pulse (about 31/2 volts in the positive direction) that grid current hows. This is arranged as follows. Valve VI amplies the pulses by a factor of 2 or 3 and also invertsthem so that their largest amplitude is in the negative direction.

Valve V8 has its bias adjusted so that no grid current flows with the largest pulse although it may be cut oi at some part of the pulse. Bearing this condition in mind it is found possible to choose an amplification for VT and a bias for V8 to produce a gain of up to in the region of zero volts. This is sufficient to allow the smallest pulse to operate the Schmitt trigger and still give a margin of safety in case of inexact adjustment of CB. Valves V9 and Vi 0 form the Schmitt trigger circuit and the square pulse output from valve V50 shown at f in Fig. 4 is diderentiated by CIELRi, the diiferential amplified by valve Vl I and then fed on to a cathode follower formed by valves Vl 2 and VI 3 in parallel to give the necessary low output impedance. The waveform at the grid of valve VII is shown at g and that at the cathodes of valves Vl2, Vl3, is shown at h in Fig. 4.

The grouping discriminator GD allows any desired fraction of the pulse range of amplitudes to be selected and amplified for analysis in the circuits already described. In the unit GD, VI is the discriminator valve and is a cathode follower eut off by a negative bias adjustable from 0 to 100 volts. Positive pulses are applied to this valve, ranging in amplitude from 3 to 100 volts. Obviously no current will iiow through Vl until the instantaneous pulse amplitudes exceeds the bias, and thereafter the current, and so the change in anode potential will be almost immediately linearly proportional to the amount by which the input pulse exceeds this bias (by virtue of the cathode follower action). Any negative pulses taken off from the anode thereafter only represent that part of the input pulse which exceeds the bias. This pulse is then passed on to V2 which inverts it and limits any pulse to 6. about 25 volts amplitude` V3i then` amplies-these pulses byv a factor of exactly 5.,. the limiting ac.- tion of V2y only being necessary to prevent overloading of V3. V4 and V5 form a cathode-follower output stage to give a low output: m1- pedance. Thus if the,y input pulses: rangev from 3A tov 100 volts', and the group discriminator bias is B, those pulses from B to B-i-25 voltsv are rep@ resented in the output by pulses from to 125 volts, of which; those from 0 tov 100 are handled by the.- analyser units.. This means that' input pulses fromV B to B+ have been analysed into 20. groups. The factor 5 for the amplication in this discriminator was chosen to allow the total input range from 0 to 100 to be analysed into 100 groups, it was estimated that a signal to noise ratio of greater tha-nl00:1 wouldv be rare. Of those. 100 g1'oups,.9'7 or so would be realised in practice'as the trigger pulse unit will not. func'- tion on a pulse smaller than 3 to i volts.

I claim:

l. A pulse analyser comprising an input circuit forl pulses to be analysed, a pulse generator connected to said input circuit to generate pulses of uniform amplitude and waveform coincident with the peaks of the input pulses-to be analysed, a plurality of dscri'minator circuits connected to said niput circuit, each of said discriminator circuits being adjusted to' provide output pulses in response to the' occurrence of: input pulses having anamplitude greater than a given voltage, the responsive voltage of successive discrimina-tor circuits being successively higher, a gate circuit normally non-conductive to the passage or pulses therethrough,'said gate circuit having an input element together with two control elements connected respectively to the pulse output of' two successive discriminator circuits to thereby be made conductive when a pulse from one discriminator is received in the absence of a pulse from the successive discrimi'nator, means to connect the uniform pulse output from said generator means to the input element of said gate circuit, and means connectedA to the output of said gate circuit to indicate the passage'of pulses therethrough.

2. A pulse analyser comprising anv input circuit for pulses to be analysed', a pulse generator connected to said input circuit to generate` pulses of uniform amplitude and waveform coincident with the peaks of the input pulses to be analysed, a plurality of discriminator circuits connected to said input circuit, each of said discriminator circuits being adjusted. to provide output pulses in response to the occurrence of input pulses having an amplitude greater than agiven voltage, the responsive voltage or successive idiscriminator circuits being successively higher, a plurality of gatecircuits-normally non.- conductive to the passage ci pulses there-through, each of said gate circuits having an input eleY ment together with two control elements connected respectively to the pulse output of two successive discriminator circuits to thereby be made conductive when a pulse from one discriminator is received in the absence of a pulse from the successive discriminator, means to connect the uniform pulse output from said generator means to the input elements of all or" said gate circuits, and means connected to the output of each of said gate circuits to indicate the passage of pulses therethrough.

3. A pulse analysercornprising. an input circuit for pulses to be analyseda pulse generator-con.- nectedtosaidiinput circuit to'generatezpulses of amplitude and; waveform. coincident with the peaks of the input pulses to be analysed, two of discriminator circuits connected to said input circuit, each of said discriminator circuits being adjusted to provide output pulses in response to the. occurrence of input pulses having an amplitude greater than a given voltage, the responsive voltage of said discri'rninatorcircuits being' at the upper and lower limits of a given. voltage range',A a gate. circuit normally nouv-conductive t0 the passage of pulses therethrough, said gate circuit having an input elementi together with two control elements connected respectively'to the pulse output of the two discriminator circuits to therebybe made conductive when a pulse from one discriminator is received in the absence of a pulse from the other discriminator, means to connect the uniform pulse output from said generator means to the input element of said' gate circuit, and means connected to the output of said gate circuit to indicate the passage of' pulses therethrough, the pulses generated by said generator and applied tothe input elements of said gate circuit occurring substantially7 coincidental with the pulse output applied to the control elements` 4. A pulse analyzer comprising an input circuit for pulses to be analysed, a pulse generator connected to said input' circuit to generate pulses of uniform amplitude and waveform coincident with the peak-s of the input pulses to be analysed, a piuralitv of discriminator circuits connected to said input circuit, cach of said discriminator circuits being adjusted to provide output pulses in response to the occurrence of input pulses having an amplitude greater than a given voltage, the responsive voltage of successive disoriininator circuits being successively higher, a plurality of gate circuits normally non-conductive to the passage of pulses therethrough, each of said gate circuits havingr an input element together with two control elements connected respectively to the pulse output of two successive discriminator circuits to thereby be made conductive when a pulse from one discriminator is received in the absence of a pulse from the successive discriminator, means to connect the uniform pulse output from said generator means to the input elements of all of said gate circuits, and means connected to the outputs oi each of said gate circuits to indicate the passage of pulses therethrough, the pulses generated by said generator and applied to the input elements of said gate circuits occurring substantially coincidental with the pulse output applied to the control elements.

5. A pulse analyser comprising an input terminal for pulses tobe analysed, a pulse generator having diiierentiating and limiting elements and connected to said input terminal to generate pulses of uniform amplitude and waveform each coincident with the peak of' a respective one of the input pulses to be analysed, a gate circuit having an input element and a control element, said gate circuit being normally non-conductive, two discriminator means connected to said input terminal and responsive to the upper and lower limits of a predetermined voltage amplitude range of the pulses to be analysed, the output oi said discriminator means being connected to the control element of said gate circuit to render said gate circuit conductive upon the occurrence of pulses in the given voltage amplitude range, a connection from the output of said generator to the input element of said gate circuit, and a pulse rate counter connected to the output of said gate circuit.

6. The invention of claim 5 wherein the discriminator circuits are each comprised of a pair of vacuum tubes direct-coupled in a trigger pair circuit with a common cathode resistance.

7. A pulse analyser comprising an input circuit for pulses to be analysed, a, pulse generator connected to said input circuit to generate pulses of uniform amplitude and waveform coincident with the peaks of the input pulses to be analysed, a plurality of discrimina-tor circuits connected to said input circuit, each of said discriminator circuits being adjusted to provide output pulses in response to the occurrence of input pulses having an amplitude greater than a given voltage. the responsive voltage of successive discriminator circuits being successively higher, the output pulses of each of said discriminators beiner Ormed in simultaneous positive and negative versions, a plurality of gate circuits normally non-conductive to the passage of pulses therethrough, each of said gate circuits having an input element together with two control elements, one of said control elements being connected to the positive version of the pulse output of one discriminator and the other of said control elements being conl nected to the negative version of the pulse output of the succeeding discrimina-tor circuit whereby said gate circuit is made conductive only in the presence of a positive pulse on said rst control element, means to connect the uniform pulse output from said generator means to the input elements of all of said gate circuits, and means connected to the outputs of each of said gate circuits to indicate the passage of pulses therethrough.

8. A pulse analyser comprising an input terminal for pulses to be analysed, a pulse generator connected to said terminal to generate pulses of uniform amplitude and waveform each coincident with the peak of a respective input pulse. two amplitude discriminator circuits also connected at their inputs to said terminal and responsive to the upper and lower limits respectively of a predetermined Voltage amplitude range of the pulses to be analysed, a gate circuit having input, control, and output connections, the outputs of the pulse generator and discriminator circuits being connected to said input and control connections respectively, and a pulse rate counter connected to said gate circuit output connection whereby uniform pulses from said generator corresponding to pulses in a predetermined amplitude range are fed to and are counted by 'said counter.

MICHAEL J. POOLE.

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